Alex McColgan

The sulphide needed to produce greigite could have come from volcanic gases leaking into groundwater.

But that means the sulphide would have had to migrate from a hot volcanic system into a much cooler environment, and there's been no evidence for such volcanic or hydrothermal sources nearby.

Another possibility is that sulfate in the rocks was reduced to sulfide through reactions with organic matter.

But unless temperatures exceed 150 to 200 degrees Celsius, these reactions would be very slow and require a huge amount of energy, making them unlikely.

and studies of the rocks around this area have shown no evidence of high temperatures.

So there's no way the surrounding environment could have got hot enough to reduce sulphate and form greigite.

Another possible explanation is acidity.

Both iron ions and sulfate ions dissolve much more readily in water under acidic conditions than they do under neutral conditions, making them much more prone to reduction through purely chemical reactions.

If the water on Mars was more acidic than we anticipated, that could have caused the spots Percy saw.

Perhaps these spots were just the result of chemical processes on an alien planet, nothing more.

But then, Percy spotted this little green mineral.

Nestled near the sample site, a small rock of olivine knocked the acidic water hypothesis on its head.

Olivine is the fastest-weathering silicate mineral.

Unlike other silicone structures like silicon dioxide, for example, olivine doesn't have strong silicon-oxygen-silicon bonds.

Instead, it's made up of negatively charged silicate ions held together by the electrostatic attraction with positively charged magnesium and iron ions.

In acidic conditions, these are displaced by hydrogen ions, breaking olivine down into orthosilicic acid and magnesium ions in solution.

The more acidic the environment, the more hydrogen ions there are, and the more aggressive the dissolution of olivine would be.

So the very fact that it exists rules out the possibility of acidic conditions causing the strange spots.

Science is ultimately about falsification.

It's not about proving a hypothesis true.